This invention relates to an optical head device used in reproducing a signal recorded on a compact disk, a video disk, or a like disk and in recording, reproduc an information signal as regards an optical disk, a magneto-optical disk, or the like. The compact disk, the video disk, the optical disk, the magneto-optical disk, or a like disk will herein be called an optical recording medium in general.
In the manner known in the art, the optical head device is for use in combination with an optical source for generating an optical beam along an optical axis from a source point. Being used for the optical head device, the optical beam is a coherent beam. Usually, a laser diode is used as the optical source. It is to be noted that the optical beam has an actual wavelength which may vary from a nominal wavelength with the temperature.
The optical head device comprises a focussing or imaging lens system for focussing the optical beam substantially on an optical recording medium. Back towards the lens system, the recording medium reflects the optical beam as a returning beam to make the lens system produce the returning beam as a reflected beam. It should be noted that the recording medium may not necessarily be correctly positioned relative to the lens system to give rise to a focussing error. When the recording medium is correctly positioned, the focussing error is equal to zero.
An optical grating element is alternatively referred to as an optical holographic element and is partitioned into a plurality of grating regions. It is therefore possible to understand that the grating element comprises first and second regions partitioned by a partition line, which intersects the optical axis. The first and second regions are for diffracting the reflected beam into first and second diffracted beams.
An optical detector or photodetector is segmented into a plurality of detector segments. The optical detector therefore comprises first through third segments and may or may not further comprise a fourth segment. For example, the optical detector is segmented by first and second substantially parallel segmentation lines into three segments. Alternatively, the optical detector is segmented by first and second orthogonal segmentation lines into four segments. Such an optical detector receives the first and the second diffracted beams at first and second points of convergence to produce an electric output signal.
When the optical detector consists of three segments, the first and the second points of convergence are on the first and the second segmentation lines. When the optical detector consists of four segments, the first and the second points of convergence are on one of the first and the second segmentation lines on both sides of the other segmentation line. In any event, each of the first and the second points of convergence is on one of the segmentation lines that is between two of the detector segments.
When the optical head device is used in reproducing a signal recorded on the recording medium, the output signal is used as a read-out signal. When the optical head device is used in any one of three usages described above, the output signal is used as a focussing error signal in detecting the focussing error and as a tracking error signal in detecting a tracking error which arises when the lens system does not correctly or desiredly focus the optical beam along a recording track preliminarily defined on the recording medium.
In the manner which will later be described in connection with a conventional optical head device, each of the first and the second points of convergence moves in a direction forming a certain angle with the segmentation line when the optical beam is defocussed on the recording medium. A like displacement occurs when the wavelength varies. Moreover, each point of convergence deforms asymmetrically on both sides of the segmentation line when the optical beam is defocussed. Under the circumstances, the output signal is adversely affected. More particularly, it results that each of the read-out signal and the focussing error signal has a reduced intensity and that the focussing error signal is subjected to offset. The tracking error signal is similarly degraded.